Bulk-solidifying amorphous alloys are a recently discovered family of amorphous alloys, which can be cooled at substantially lower cooling rates, of about 500 K/sec or less, than conventional amorphous alloys while still retaining their amorphous atomic structure. As a result of this reduced cooling rate, these bulk-solidifying amorphous alloys can be produced in thicknesses of 1.0 mm or more, substantially thicker than conventional amorphous alloys, which require cooling rates of 105 K/sec or more, and which can only be cast in thicknesses of about 0.020 mm. U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975 (the disclosures of which are incorporated herein by reference) disclose such bulk-solidifying amorphous alloys.
Articles made from such bulk-solidifying amorphous alloys are generally produced by permanent mold casting, and more specifically in processes which utilize metallic molds. For example, U.S. Pat. Nos. 6,021,840 and 5,711,363 (the disclosures of which are incorporated herein by reference) describe such methods utilizing die-casting techniques. In the die-casting process a molten feedstock of a bulk-solidifying amorphous alloy composition is cast into a metallic mold cavity. In such a method the metallic mold mass provides the rapid cooling necessary to form the amorphous phase during solidification.
Generally, permanent mold casting is preferred over other techniques when using bulk-solidifying amorphous alloys because this method provides a relatively high cooling rate and a high production rate. However, there are cases where permanent mold casting has certain shortcomings and, as a result, cannot be generally applied. For example, a metallic mold needs to be machined to produce shaped articles of bulk-solidifying amorphous alloys. Machining such molds is not only time-consuming, but requires a substantial financial investment in order to address the various complexities in permanent mold casting. Furthermore, even slight changes in the geometry of the mold cavity may cause major difficulties and, as such, even subtle subsequent design changes generally require machining new molds causing lost time and increased costs. These shortcomings becomes particularly acute in developing new products and prototypes, and in cases where one-of-a-kind or small runs are desired. As such, machining a new mold for each shape becomes a cost-prohibitive factor and hinders the market penetration of bulk-solidifying amorphous alloys to the rapid developments in design change.
Another potential shortcoming of permanent mold casting is the detailed replication of mold features. As the molten feedstock is fed into the mold cavity, rapid cooling takes place, which accelerates solidification and impedes the flow of the viscous fluid into the intricate details of the mold cavity. The relatively cold surface of the mold cavity and the high thermal conductivity of the metallic mold material precludes any wetting of the molten alloy to the mold surface. This matter is especially pronounced in thin sections where the thickness of the article is less than about 2.0 mm.
Accordingly, a need exists for an improved casting method for bulk-solidifying amorphous alloys, which will allow for low cost tooling, and should allow rapid implementation of design changes and complete replication of mold features, while still allowing for the formation of articles having the desired amorphous phase properties.